The intra- and intergeneric position of Propionibacterium was determined by comparing reverse transcriptase sequences of 16S ribosomal ribonucleic acid. Propionibacterium jensenii. P. thoenii and P. acidipropionici formed a phylogenetically tight cluster, while P. freudenreichii, P. acnes and P. propionicus were as remotely related among each other as each of them was to the first three species. The genus Propionibacterium represents a well defined taxon that stands isolated among other major groups of the actinomycetes. Its phylogenetic neighbours are the genera Nocardioides and Terrabacter.